The first point to understand is what a marketable order is.  Marketable orders are either market orders or buy and sell limit orders whose limit price is at or above/below the current market price.  A marketable buy limit order would have a limit price set at or above the current ask in the market.  Conversely, a marketable sell limit order would have a limit price set at or below the current bid in the market. 

Marketable orders remove liquidity from the market. 

Non-marketable orders are buy and sell limit orders in which the limit price is below/above the current market price.  A non-marketable buy limit order would have a limit price that is below the current ask in the market.  Conversely, a non-marketable sell limit order would have a limit price that is above the current bid in the market. 

Non-marketable orders add liquidity to the market.

Traders should familiarize themselves with the unit of trade convention for BRK B along with IB’s policies with respect to odd lot orders.  Due to the relatively low trading volume in BRK B, NYSE Arca (the primary listing exchange for this security) has designated it as a 10-share unit issue.  A round lot in this security is therefore set at 10 shares as opposed to the standard round lot unit of 100 shares. If your opening buy or sell order is for an amount less than 10 shares, it is considered an odd-lot.  As a matter of policy, odd-lot orders are prohibited from being routed to NYSE Arca.  In addition, only those odd-lot orders which serve to open a position and which are part of a basket order or market-on-close (MOC) order may be sent to NYSE or AMEX.   Opening orders which do not meet that criteria are typically routed to a regional exchange or an ECN where they will remain until a matching order is found.  Odd-lot orders which serve to close a position may be routed to any market center to which IBKR routes orders and can be directed either by the client or via IB SmartRouting.